Determination of Absolute Configurations from Anomalous Scattering by Oxygen

Abstract

THE determination of absolute configurations by crystallographic methods depends on the measurement of the effect of anomalous scattering on diffraction intensities¹. The effect will be most pronounced when the crystal contains atoms for which the wavelength of an absorption edge is just greater than that of the radiation being scattered, but it is not limited to such cases. Anomalous scattering results in an advance in phase angle relative to a normally scattered wave. This is usually expressed by writing the atomic scattering factor as a complex number f=f_re + i▵f″. As a result of the anomalous phase shift the intensity I(hkl) of a reflexion from a set of planes (hkl) in a non-centrosymmetric crystal will in principle differ from that of a reflexion from the “back” of these planes (indices ); this difference will be readily observable only if at least one of the atomic scattering factors has a relatively large imaginary term (i▵f″).